Couple of things:
Basements/storm shelters aren’t really a thing in the South, for various reasons.
Those people may have supplies, generators, etc. They may also be in an area where flooding/storm surge was not an issue in previous storms, so they thought they could ride it out.
There are also those who have stayed before for previous storms and think they will be fine again.
And finally, some people simply do not have the money to evacuate and feel they have no choice to stay. You would hope they would go to a shelter in that case.
I agree with you that hurricanes can give someone plenty of time to prepare and plan, but the devil is as always in the details.
I tip my hat to those who live in tornado alley, I don’t know how you do it when a threat can just materialize out of nowhere.